The Offshore Mercury

ABSTRACT This paper describes the design process in the development of a new concept in exploratory drilling tools. It examines the requirements for satisfactory design of a ship in a drilling platform and an elevating system. The design efforts are brought together to form in one unit a cohesive drilling tool - the self propelled, self-elevating drill ship - The Offshore Mercury. THE OFFSHORE MERCURY The Offshore Mercury, the industry's first self-propelled and self-elevating drill ship, presents a new concept of mobility and stability in offshore exploratory drilling. It accomplishes the heretofore impossible combination of economical mobility comparable to the self-propelled drill ship with the superior stability and drilling reliability of the jack up platform. The realization of this new concept is the result of an extensive design effort. We wish to trace some design considerations involved in producing the Offshore Mercury. The first consideration is mobility. In our phase of the industry, this means moving on the sea. Move a drilling operation at sea? Easy; mount it on a buoyant box. That is a start. Push or pull it with tug boats. We do that now essentially, but we have to depend on getting the tugs out on time and having good weather to tie the tugs on and, besides, it is expensive. Let us remove the dependence on tugs; do it ourselves and make a sizeable saving on trip insurance. We have plenty of power for the rig, so hook it up to propellers and go it alone - anytime we are ready - anywhere we want to go. There you have it - a self-propelled drill ship. Almost. Let us backtrack to the point of the buoyant box that we mounted the drilling equipment on. Basically, a box that floats works out fine for the preliminary criteria. But how about the refinements that make the whole thing workable, such as how fast is a box, and how safe is this rig in rough weather? What makes a box a ship? First we must make the box big enough to carry the drilling equipment. Experience can tell us that the drill rig will be about the same size as our rig "Orion" in the North Sea. So, we first take an Orion-shaped box and look at the speed. Not too good at 3 to 4 knots even with those big tugs pulling all out. Must be something to do with the shape of the bow. "Orion" is blunt in the bow. Let us add some shape to the bow to make the water flow by more easily. This was a problem for the naval architects and the model test tanks. They took some known relationships and came up with a double curvature surface which should suit the problem. They then put the idea into a wooden model and placed it in the test tank. Based on the results of this test, they predicted an obtainable speed of about 8 knots. This is a big improvement over Orion and her tugs.